Ho-doped fiber lasers are of interest for high energy laser applications because they operate in the eye safer wavelength range and in a window of high atmospheric transmission. Because they can be resonantly pumped for low quantum defect operation, thermal management issues are anticipated to be tractable. A key issue that must be addressed in order to achieve high efficiency and minimize thermal issues is parasitic absorption in the fiber itself. Hydroxyl contamination arising from the process for making the Ho-doped fiber core is the principal offender due to a combination band of Si-O and O-H vibrations that absorbs at 2.2 μm in the Ho<sup>3+</sup> emission wavelength region. We report significant progress in lowering the OH content to 0.16 ppm, which we believe is a record level. Fiber experiments using a 1.94 μm thulium fiber laser to resonantly clad pump a triple clad Ho-doped core fiber have shown a slope efficiency of 62%, which we also believe is a record for a cladding-pumped laser. Although pump-power limited, the results of these studies demonstrate the feasibility of power scaling Ho-doped fiber lasers well above the currently-reported 400-W level.<sup>1</sup>
Anti-Stokes fluorescence cooling has been demonstrated in a number rare earth doped materials. Ytterbium doped
oxides and fluorides, such as ZBLAN, YLF, and YAG, were the first materials to exhibit cooling.<sup>1,2,3</sup> These materials
were originally developed as laser gain media and fluorescence cooling was eventually incorporated into the 1μm lasers
to reduce detrimental thermal loading.<sup>4</sup> Anti-Stokes cooling can offset quantum defect heating allowing laser power to be scaled to very high average powers.
Since the early work in ytterbium, fluorescence cooling has been demonstrated in both erbium and thulium doped
materials.<sup>5,6</sup> These materials were also initially developed as lasing media and their fluorescence cooling could be used to increase laser powers at 1.5μm and 2.0μm. In this study we examine the radiative efficiency of holmium and ask the question, “Can anti-Stokes fluorescence cooling be extended beyond 2μm?”
We are investigating materials for direct blue solid-state lasers assuming UV excitation with GaN based laser diodes.
Room temperature spectroscopy is reported relevant to a proposed quasi-three level laser from the <sup>4</sup>F<sub>9/2</sub> level in trivalent
dysprosium. Modeling based on these measurements suggests this is a promising new laser transition.
Rare earth doped ternary lead salts are being studied for use as mid-IR laser materials. We summarize progress at the Naval Research Labs on the production and evaluation of this important class of solid-state laser.